Engagement of the multicomponent antigen receptor in T cells (TCR) results in rapid activation of a protein tyrosine kinase pathway. To fully understand the function of this pathway, the kinase and its substrates must be characterized. A major TCR- associated protein tyrosine kinase is ZAP-70, a protein that binds to the activated TCR. Work from our laboratory demonstrated that the tandem SH2 domains in this kinase are responsible for its interaction with TCR subunits following their tyrosine phosphorylation. We have now shown that these SH2 domains bind directly to the tyrosine phosphorylated tyrosine-based activation motifs (TAMs) found in the cytoplasmic domains of the CD3 and TCR- chains. A peptide with the sequence of the third TAM of the TCR- chain was synthesized with the two tyrosines replaced with difluorophosphonomethyl phenylalanine (F2Pmp), a phosphatase-resistant analog of phosphotyrosine. This reagent, (F2Pmp)-TAM zeta3, when introduced into T cells blocks ZAP-70 assembly with the TCR, tyrosine phosphorylation and activation of ZAP-70, and substrate phosphorylation. The reagent thus serves as a prototype for drugs that may ultimately be used therapeutically to block T cell activation. Additional studies focus on two substrates of tyrosine kinases in T cells. They are VCP, an hexameric ATPase and p120cbl, a proto-oncogene, which may be involved in the ras pathway in T cells. Abnormal tyrosine phosphorylation of the cell cycle kinase, p34cdc2, has been detected in T cells that are infected with human immunodeficiency virus (HIV). Abnormal regulation of this enzyme correlates with the cell cycle arrest at G2/M and programmed cell death that can be observed with HIV infection.